The problem with having vents in your crawl space is that in the winter they let in cold air which travels directly into your home. Floors above can become very cold due to cold air traveling between the floors and the insulation. In the summer time the vents let in hot air which mixes with the cool conditions of the crawl space. This causes condensation to form on everything throughout the crawl space. When this happens, microbial growth occurs which can attract insects and cause wood rot.